Magnetic resonance imaging (MRI) is a very powerful tool in research and diagnostics. It comprises immerging a body in a static magnetic field B0 for aligning nuclear spins thereof; exposing it to a transverse radio-frequency pulsed field B1 (excitation sequence) at a resonance frequency known as the “Larmor frequency” for flipping said nuclear spins by a predetermined angle; and detecting a signal emitted by flipped nuclear spins, from which an image of the body can be reconstructed.
The book by E. M. Haacke, R. W. Brown, M. R. Thompson and R. Venkatesan “Magnetic Resonance Imaging”, Wiley & Sons, New York (1999) describes several conventional MRI techniques; in particular, chapter 18 deals with fast imaging methods, which are widespread e.g. in medical practice.
It is known (see e.g. the above-referenced book by E. M. Haacke, chapter 18, paragraph 1) that the intensity of the MRI signal depends on the spin-flip angle, and reaches a maximum for a spin-flip angle value, known as “Ernst angle” for spoiled gradient echo sequences, which is a function of physical properties of the body to be imaged (relaxation time T1) and of the repetition period of the radio-frequency excitation pulse.
Unfortunately, the Ernst angle is not the same for all the tissues of a body. For example, considering a pulse repetition time of 80 ms, the Ernst angles for protons in muscle and fat tissues are of about 19° and 37° respectively. Therefore, according to the prior art, a same excitation sequence cannot flip the spins of nuclei in different tissues at their respective Ernst angle.
As a consequence, the MRI signal intensity, and therefore the corresponding signal-to-noise ratio, cannot be optimized by conventional excitation sequences for several tissues.
Instead of optimizing the signal intensity, one may want to optimize the contrast between MRI signals radiated by nuclei belonging to different tissues. But even achieving this goal is hindered by the above-discussed phenomenon.
Actually, the Larmor frequency of nuclei of a given chemical specie (typically, Hydrogen) depends on its chemical environment; as a consequence, nuclei (e.g. protons) belonging to different tissues are flipped by slightly different angles by a same excitation sequence, i.e. they respond differently under the influence of an RF pulse. This effect allows independent control of spin flip angles of nuclei belonging to different tissues by using very “soft” pulses whose shapes yield in the Fourier domain the desired excitation around the frequency of interest: see e.g. the paper by M. Steffen, L. Vandersypen and I. Chuang, “Simultaneous soft pulses applied at nearby frequencies”, Journal of Magnetic Resonance 146, 369-374 (2000). However, use of such pulses implies long acquisition times and a high specific absorption rate (SAR), which makes this technique unsuitable for in-vivo imaging.